1. Field of the Invention
The present invention relates to a system and a method for determining at least one physical quantity, usable preferably in active generation of corresponding depth and reflectivity images by means of a laser, and utilization thereof for environment detection.
2. Discussion of Background
It is known to use an active AMCW, or amplitude modulation continuous-wave laser measurement system, for range finding and/or reflectivity measurement. In the case of such a measuring system there exists, for example, the possibility of creating a sinusoidal transmission signal having the form TRM(t)=sin(.omega..sub.1 t) e.g. by means of a semiconductor laser diode and emitting it over a to be measured signal path of the geometrical length D.
Owing to the signal path, the reception signal having the form REC(t)=B sin(.omega..sub.1 t-.phi.) exhibits both an attenuation and a phase shift with respect to the original transmission signal TRM(t). The reception signal may, for example, be received by means of an avalanche photodiode.
It is an essential aspect of such a system to determine the existing phase shift as well as the existing attenuation with high precision, i.e. a relative error of approximately 0.01%, and with a very high measurement rate, i.e. up to 1.times.10.sup.6 measurements/second. Herein the reception signal may have high signal dynamics or attenuation of up to 80 dB (1:10,000).
The geometrical length D of the measuring sticks and of the signal path in the system may be calculated directly for example by means of the following equation (1). ##EQU1##
wherein, c designates the propagation velocity of the transmission signal TRM(t), .omega..sub.1 designates the measurement frequency used, .phi. designates the phase shift of the reception signal REC(t) with respect to the transmission signal TRM(t), and D designates the geometrical length of the signal path.
The following is a description of a principle of measurement used in the above described system and method.
The original sinusoidal transmission signal TRM(t)=sin(.omega..sub.1 t) is compared to the reception signal REC(t)=B sin(.omega..sub.1 t-.phi.) received at the end of the signal path, which is now attenuated and phase shifted relative to the transmission signal TRM(t). In order to obtain the phase shift .omega. of interest, the following equation (2) is used, which is obtained by application of the general mathematical interrelation sine sin .beta.=1/2[cos(.alpha.-.beta.)-cos(.alpha.+.beta.)]. EQU sin(.omega..sub.1 t).multidot.B.multidot.sin(.omega..sub.1 t-.phi.)=B/2[cos(-.phi.)-cos(2.omega..sub.1 t-.phi.)] (2)
As can be seen from the left-hand member of equation (2), the original transmission signal TRM(t) is multiplied by the reception signal REC(t) to be evaluated. Following this multiplication, the double-frequency signal component (cos(2.omega..sub.1 t-.phi.)) resulting from the multiplication is filtered out. In this way the phase angle to be determined is indirectly available in the value B/2 cos(-.phi.). This term, however, contains two unknown quantities (which are to be determined), i.e., B and .phi., so that the following equation (3) is required which is obtained by application of the general mathematical interrelation sine cos .beta.=1/2[sin(.alpha.-.beta.)+sin(.alpha.+.beta.)]. EQU cos(.omega..sub.1 t).multidot.B.multidot.sin(.omega..sub.1 t-.phi.)=B/2[sin(-.phi.)+sin(2.omega..sub.1 t-.phi.)] (3)
Here, as well, the double-frequency signal component (sin(2.omega..sub.1 t-.phi.)) resulting from the multiplication is filtered out, so that ultimately the two intermediate results B/2 cos(-.phi.) and B/2 sin(-.phi.) containing the two unknown quantities B and .phi. are obtained. By applying the following equations (4) and (5) it is now possible to calculate the two values B and .phi. it to be determined. ##EQU2##
Herein B* and .phi.* designate the calculated values for differentiation from the physical measurement values B and .phi..
It is now possible to calculate the geometrical length D of the signal path by applying the value .phi.* thus determined in accordance with equation (1), and to calculate the intensity, or reflectivity, of the reception signal REC(t) by applying the attenuated amplitude thus determined of the reception signal REC(t).
Although the above described principle of measurement is absolutely exact in mathematical terms, there nevertheless result the following problems in technical implementation.
The signals to be multiplied by each other are located within a range of several tens of MHZ. This multiplication is generally performed by means of analog mixers. Such analog mixers are, however, not absolutely linear and their parameters are moreover temperature-dependent. This brings about errors in multiplication which falsify the final result, i.e., induce errors of measurement.
Moreover the signal cos(.omega..sub.1 t) in equation (3), which is necessary for calculation, is generated from the original transmission signal TRM(t)=sin(.omega..sub.1 t). The analog phase shifter usually employed for this purpose is, however, not an ideal phase shifter and accordingly causes equally temperature-dependent amplitude and/or phase errors resulting in errors of measurement.
Finally, the calculations required for obtaining B* and .phi.* are performed in large-scale integrated digital signal processors, which requires that prior to these calculations the two intermediate results B/2 cos(-.phi.) and B/2 sin(-.phi.) obtained must be filtered and subsequently converted from an analog signal into a corresponding digital signal by means of an analog-to-digital converter. Due to the fact that these two intermediate results are processed through different, non-ideal filters and analog-to-digital converters, in turn temperature-dependent errors of measurement are introduced.